1. Field of the Invention
The present invention relates to a process for the preparation of polyarylene thioethers having an elevated melt crystallization temperature, which polyarylene thioethers are advantageously used in, among others, injection molding.
2. Description of the Prior Art
Among thermoplastic resins, a polyarylene thioether is a heat-resistant resin having a relatively high crystallization speed. Accordingly, it is expected that this polymer will be applied to injection molding for which a high crystallization speed is required.
Most heretofore marketed polyarylene thioethers are products in which the apparent melt viscosity is increased by subjecting a product having a relatively low melt viscosity (that is, a low molecular weight) to curing, that is, a high temperature treatment in the presence of oxygen or the like to carry out a reaction such as crosslinking or branching.
In a resin having a melt viscosity increased by curing, in general, the melt crystallization temperature (Tmc) (the temperature at which crystallization is caused when the molten resin is cooled) is much higher than the melt crystallization temperature of the untreated resin, and therefore, the crystallization speed is greatly increased. Accordingly, only in view of the crystallization behavior, it can be said that a cured polyarylene thioether is suitable for injection molding. However, although the crystallization behavior is appropriate, the cured polyarylene thioether is defective in that coloring is extreme, and, since it is highly crosslinked and branched, its mechanical strength is insufficient.
We have previously proposed a polymerization process in which a large amount of water is added midway in the polymerization reaction, and, simultaneously, the polymerization temperature is elevated (a linear polyarylene thioether having a high molecular weight is economically advantageously prepared by water-adding two-staged polymerization). See Japanese Patent Application No. 126725/84, which is now Japanese Patent Laid-Open Application No. 7332/1986.
Since a polyarylene thioether prepared according to this process ordinarily has a sufficiently high melt viscosity, the resin can be directly applied to melt processing such as extrusion molding or compression molding without curing, and, since curing is not carried out, coloring is substantially controlled. Moreover, this resin has excellent mechanical strength because it has a linear structure. However, it was found that this high-molecular-weight polyarylene thioether developed by us is still inadequate as a resin material for injection molding. More specifically, Tmc is somewhat too low for application to injection molding, and under certain processing conditions for solidifying the molten resin, it sometimes happens that crystallization is insufficient. It is desirable that polyarylene thioethers for injection molding have Tmc of at least 220.degree. C., preferably at least 225.degree. C.
Generally, the higher the Tmc is, the higher is the crystallization speed when the molten resin is cooled and solidified.